The 1st strip must be plumb. Do not start in a corner - corners are never plumb. Start about 1/2 short of the width from a corner. Use a 4-5 foot level and draw a thin line. This is where the 1st strip will go.
Another hint: prime the walls with shieldz first. Shieldz eliminates the need for sizing and helps hanging the paper. You get less stretching of the paper.